Process for manufacturing electronic circuits

ABSTRACT

A process for manufacturing electronic circuits, according to which soldering can be carried out without using flux by applying a metal surface treatment procedure which allows oxide film, organic matters, carbon or the like on the surface of metal to be easily removed without using a complex process nor unfavorably affecting electronic devices or circuit substrates. A process of connecting an electronic device and a circuit substrate by means of solder comprises the steps of irradiating the solder with a laser beam to clean the solder, aligning and mounting the electronic device on the circuit substrate, and hot-melting said solder in a low-oxygen content atmosphere to bond the electronic device and the circuit substrate.

BACKGROUND OF THE INVENTION

This invention relates to a process for manufacturing an electroniccircuit wherein an electronic device or component (part) such as asemiconductor integrated circuit (LSI) is connected to a circuitsubstrate, especially by soldering without using flux.

Soldering between a circuit substrate and a semiconductor integratedcircuit (LSI) or the like has required that the surfaces of the metalsin question to be joined should be kept clean and free from any materialhindering wettability.

Plating has also required that the surface of the metal in question tobe plated should be kept clean so that an oxide film or layer or thelike does not exist thereon.

When an Au wire or Au ribbon or the like is to be bonded on the surfaceof the metal in question by an ultrasonic heat pressure technique, anoxide film on the surface of the metal in question is also troublesomeand therefore the surface of the metal in question must be kept clean.

The materials hindering wettability of solder include oxides, chlorides,sulfides, carbonates, various types of organic compounds, etc.Particularly, the most serious obstacle in the processes such assoldering, plating or ultrasonic heat pressure bonding of an Au wire orAu ribbon is an oxide film present on the surface of the metal inquestion such as solder, nickel (Ni), nickel alloys (alloys of nickelwith other substances).

An oxyde film is usually chemically dissolved by flux and converted intoa liquid compound. Thus, the surface of the metal in question and metalatoms of solder get an opportunity to directly collide with each otherto form a metallic bonding state sharing an outer electron shell, sothat the metal in question and solder can be alloyed. However, fluxresidues still remain on the surface and must be cleaned off.

The presence of an oxide film hinders plating. For example, an oxidefilm acts as an insulating (dielectric) film to prevent electricconduction therethrough necessary for electroplating, which is a typicalexample of plating, and thus hinders such electroplating.

Oxide film also hinders replacement plating by preventing the reactionbetween the surface of the metal in question and a plating liquid.

In these plating processes, oxide film should be removed by liquidtreatment with hydrochloric acid or the like. However, residues stillremain and are largely responsible for lowering the bonding reliability.Residues have been conventionally cleaned off with chlorofluorocarbons.

Recently, a technology has been developed to omit the step of cleaningoff flux residues by using abietic acid (rosin) which leaves a lesseramount of residues as flux and a minor amount of adipic acid or thelike. However, the bonding reliability is insufficient.

This technology is described in detail in "Alumit Technical Journal 19"(1992) and "Action Mechanism and Problems of Cleaning-Free Flux" byKubota of Japan Industrial Technology Development Research Institute,Co.

A glazing method has also been proposed, which provides highlycorrosion- and abrasion-resistant materials with very fine homogeneoustexture or amorphous structure by irradiating metallic materials, steelmaterials, carbides or the like with laser beam. This glazing method isused for processing metallic materials to be subjected to hightemperatures and high pressures such as materials for automobileturbines, and discussed in, for example, "Laser Processing, SecondSeries", by A. Kobayashi, pp. 164, published by Kaihatsusha.

A known method for removing oxide film on the surfaces of metals withoutusing flux or hydrochloric acid employs argon sputtering.

Japanese Patent Application Laying Open (KOKAI or UnexaminedPublication) No. 63-97382 discloses a method for providing a highlyadhesive pinhole-free coating film by plating a surface of a metallicworkpiece, roughened by blasting with an alloy element, and thereaftermelt processing the plated layer by irradiation with a laser beam.

Further, Japanese Patent Application Laying Open (KOKAI) 62-256961discloses a method for preparing a surface-treated layer with goodcorrosion resistance and solderability by forming an anodized layer onthe surface of a base material formed of aluminium or alloys thereof.

SUMMARY OF THE INVENTION

As a result of reviews and studies of the above prior art, the inventorshave found the following problems.

(1) The method for removing oxide film by means of flux before solderinga circuit substrate and an integrated circuit involves the problem ofnecessarily requiring the step of cleaning off flux residues. Acid orthe like remaining as residues may also cause the corrosion of metal.

In addition, a drying step is indispensable after cleaning.

(2) The method for removing oxide film by argon sputtering involves theproblem of requiring a treatment in vacuum, with the result that theequipments can hardly be controlled and electronic devices or activeelements of electronic devices may be unfavorably affected by argonsputter.

(3) The laser beam-irradiated glazing method and the laser processingmethod disclosed in Japanese Patent Application Laying Open (KOKAI)63-97382 will disadvantageously cause oxide film to grow duringsolidification of the metal surface because both methods force the metaltexture on the surface to melt by a high energy laser beam in order tohave abrasion resistance or denseness on the metal surface.

The surface treatment method disclosed in Japanese Patent ApplicationLaying Open (KOKAI) 62-256961 can not be applied because it does notdisclose the technique for removing oxide film.

The object of this invention is to solve the above problems of the priorart and provide a process for manufacturing electronic circuits,according to which soldering can be carried out without using flux byapplying a metal surface treatment procedure which allows oxide film,organic matters, carbon or the like on the surfaces of metals to beeasily removed without a complex process and without unfavorablyaffecting electronic devices, components or circuit substrates.

According to this invention, the above object is attained by a procedureof connecting an electronic device or component and a circuit substrateby means of solder, comprising the steps of irradiating said solder witha laser beam to clean said solder, aligning or registering, and mountingsaid electronic device on said circuit substrate, and hot-melting saidsolder in a low-oxygen content atmosphere to bond said electronic deviceand said circuit substrate with each other.

In this procedure, the surface of a metal such as solder is irradiatedwith a laser beam having a lower energy than the energy necessary tochanging the texture of the surface of the metal. Thus, only the bondsbetween metal atoms and oxygen atoms on the surface of the metal arebroken or released by the energy of the laser beam to remove oxide filmas well as organic matters, carbon, etc. on the surface while the metaltexture on the surface remains unmolten.

Oxide film on the surface of the metal can successfully be removedwhether the laser beam irradiation takes place in any of the atmospheressuch as air or He gas, or in a vacuum.

Then, the electronic device or component and circuit substrate to beconnected by solder are aligned or registered by a pre-setting liquid,and thereafter they are soldered together by hot-melting the solder fromwhich has been removed oxide film in a low-oxygen content atmosphere.Thus, good soldering can be achieved without oxidizing solderedsurfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view for illustrating a metal surface treatmentprocedure according to Example 1 of this invention.

FIG. 2 is a sectional view for illustrating a variation or modificationof Example 1 wherein the surfaces of solder bumps on a semiconductorintegrated circuit (LSI) or the like are irradiated with a laser beamvia a lens or mirror, instead of the solder layer of FIG. 1.

FIG. 3 is a scanning electron micrograph of the surface of the solderlayer before irradiation with a laser beam according to the Example 1.

FIG. 4 is an enlarged photograph of FIG. 3.

FIG. 5 is a scanning electron micrograph of the surface of the solderlayer after being irradiated with a laser beam according to Example 1.

FIG. 6 is an enlarged photograph of FIG. 5.

FIG. 7 is a graph plotting the relation between the oxide film level orproportion (%) on the Sn--Pb surface as ordinate and the laser beamirradiation energy density per pulse (J/cm²) as abscissa in Example 1.

FIG. 8 is a graph plotting the oxide film level (%) on the Sn--Pbsurface as ordinate and the irradiation cycle number (number ofirradiations) at a fixed energy density of 1.5 (J/cm²) as abscissa inExample 1.

FIG. 9 is a sectional view showing an example of an electronic circuitsoldered according to Example 1.

FIG. 10 is a sectional view showing another example of an electroniccircuit soldered according to Example 1.

FIG. 11 is a sectional view for illustrating a metal surface treatmentprocedure according to Example 2 of this invention.

FIG. 12 is a graph plotting the relation between the thickness of theoxide film (nm) formed on the nickel layer as ordinate and the laserbeam irradiation energy density (J/cm²) as abscissa when the laser beamirradiation cycle number for the same zone of the nickel layer is fixedin Example 2.

FIG. 13 is a graph plotting the relation between the thickness of theoxide film (nm) formed on the surface of the nickel layer as ordinateand the laser beam irradiation cycle number for the same zone of thenickel layer as abscissa when the laser beam irradiation energy densityis fixed in Example 2.

FIG. 14 is a sectional view for illustrating a process forking anelectronic device according to Example 3 of this invention.

FIG. 15 is a configurative sectional view of an electronic device towhich the anti-reoxidizing means has actually been applied in Example 3.

FIG. 16 is a sectional view showing an example wherein an electronicdevice and a nickel (Ni) layer or nickel alloy layer on a ceramicsubstrate are directly electrically connected by solder without using aninput/output (I/O) pin of FIG. 15.

FIG. 17A is a plan view for illustrating a process for making anelectronic circuit according to Example 4 of this invention.

FIG. 17B is a sectional view taken along a line XVIIB--XVIIB of FIG.17A.

FIG. 18 is a sectional view showing the configuration of a circuitsubstrate on which has been pre-set an electronic device to be solderedby a manufacturing apparatus according to an embodiment of thisinvention.

FIG. 19 is a perspective view showing the configuration of an apparatusfor manufacturing electronic circuits according to an embodiment of thisinvention.

FIG. 20 is a sectional view showing an inside of a treating vessel.

FIG. 21 lists examples of the liquid used for re-setting an electronicdevice.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of this invention will now be described indetail by way of example with reference to the accompanying drawings.

EXAMPLE 1

FIG. 1 is a sectional view for illustrating a metal surface treatmentprocedure or process or method according to Example 1 of this invention,which shows a ceramic substrate 1, a metallized layer 2, a solder layer3a, oxide film or layer 4, laser beam 5, a lens 6, and a mirror 7.

According to the metal surface treatment procedure of the embodiment 1,the oxide 4 grown (or residues of organic matters, carbon, etc.) on thesurface of the solder layer 3a on the surface of the metallized layer 2formed on the top of the ceramic substrate 1 is removed as shown in FIG.1.

The metallized layer 2 is made of, for example, a titanium (Ti), nickel(Ni), or nickel alloy film.

In order to remove the oxide 4 (or residues of organic matters, carbon,etc.) on the surface of the solder layer 3a, the surface of the solderlayer 3a is irradiated with the laser beam 5 via the lens 6 and themirror 7. The oxide 4 is thus removed.

FIG. 2 shows a variation or modification of Example 1, wherein thesurfaces of solder bumps 3b on a semiconductor integrated circuit (LSI)or the like are irradiated with laser beam 5 via a lens 6 and a mirror7, instead of the solder layer 3a of FIG. 1.

The laser beam 5 used in Example 1 has a lower energy than the energyrequired to change the metallic texture or structure of the solder layer3a or solder bumps 3b. More specifically, it has an energy which ishigher than the bonding energy between Sn (tin) atoms and O (oxygen)atoms on the surface of the solder layer 3a or solder bumps 3b but lowerthan the bonding energy between Sn--Pb atoms.

When the solder layer 3a or a solder bump 3b is irradiated with suchlaser beam 5, only the bonds of Sn--Pb atoms with O atoms on the surfaceare broken or released by the energy of the laser beam 5 while thesolder on the surface remains unmolten. The oxide film 4 on the surfaceof the solder layer 3a or solder bumps 3b is thus removed. Organicmatters, carbon, etc. on the metal surface are also removedsimultaneously.

Since the main purpose of the irradiation with the laser beam 5 hereinis to break or release the bonds of Sn--Pb atoms with O atoms on thesurface, the laser beam 5 is preferably a pulsed laser beam with a pulsewidth of 1 μs or less, for example.

Assuming that the bonds between Sn--Pb atoms and O atoms on the surfaceare broken by a pulsed laser beam with a pulse width of 1 μs or less,the laser beam 5 is preferably an excimer laser beam with a shortwavelength (high photon energy), for example.

The oxide film 4 on the surface of the solder layer 3a or solder bumps3b could be conveniently removed whether the irradiation with the laserbeam 5 took place in any of the gaseous atmosphere such as air or He gasor in vacuum.

FIG. 3 is a photograph of the surface state or condition of the solderlayer 3a observed by a scanning electron microscope before irradiationwith a laser beam, and FIG. 4 is an enlarged photograph of FIG. 3.

These photographs show black residues of organic matters or carbon, etc.on the surface of the solder layer 3a.

FIG. 5 is a photograph of the surface state or condition of the solderlayer 3a similarly observed by a scanning electron microscope afterirradiation with laser beam, and FIG. 6 is an enlarged photograph ofFIG. 5.

These photographs show that residues of organic matters, carbon, etc.have been substantially wholly removed.

FIG. 7 is a graph plotting the relation between the oxide film level orproportion (%) on the Sn--Pb surface as ordinate and the laser beamirradiation energy density per pulse (J/cm²) (laser beam irradiationenergy per unit area) as abscissae.

FIG. 7 shows that the residual oxide film level is lower than theuntreated oxide film level when the laser beam irradiation energydensity ranges from 0.5 J/cm² to 4.0 J/cm², and especially reaches theminimum when the laser beam irradiation energy density is 1.5 J/cm².

The oxide film level herein is the oxygen content measured by energydiffusion X-ray spectroscopy (EDX).

FIG. 8 is a graph plotting the oxide film level or proportion (%) on theSn--Pb surface as ordinate and the laser beam irradiation cycle number(i.e. number of times of irradiation) as abscissa when the laser beamirradiation energy density is fixed at 1.5 (J/cm²).

FIG. 8 shows that the residual oxide film level is low when theirradiation cycle number ranges from 6 to 10 , and especially reachesthe minimum when the irradiation cycle number is 8.

These results prove that the oxide film level on the Sn--Pb surfacereaches the minimum and the wettability of the solder layer 3a or solderbumps 3b is improved in a case where 8 cycles of laser beam irradiationare applied at an energy density of 1.5 J/cm².

FIG. 9 shows a cross section of a main part of an electronic circuitmade by flux-free soldering an integrated circuit (LSI) 8 onto ametallized layer 2 formed on the top of a ceramic substrate 1 afterremoving oxide film on the surface of solder bumps 3b according to themetal surface treatment procedure of Example 1, and FIG. 10 shows across section of a main part of an electronic circuit similarly made byflux-free soldering after removing oxide film on the surface of solderbumps 3b under a sealing cap 9.

EXAMPLE 2

FIG. 11 is a sectional view for illustrating a metal surface treatmentprocedure or process or method according to Example 2 of this invention.

According to the metal surface treatment procedure of Example 2, anoxide 4 (or residues of organic matters, carbon, etc.) on the surface ofa nickel (Ni) layer (or nickel alloy layer) 2a formed on the top of aceramic substrate 1 is removed as shown in FIG. 11.

The nickel (Ni) layer (or nickel alloy layer) 2a is normally liable tobe oxidized so that the oxide film 4 is readily formed on the surface ofthe nickel (Ni) layer or nickel alloy layer 2a.

In order to remove the oxide 4 on the surface of the nickel layer 2a,the surface of the nickel layer 2a is irradiated with laser beam 5 viathe lens 6 and the mirror 7, in the same way as the above Example 1.

FIG. 12 is a graph plotting, by way of example, the relation between thethickness (nm) of the oxide film 4 formed on the nickel layer 2a asordinate and the laser beam irradiation energy density (J/cm²) (laserbeam irradiation energy per unit area) as abscissa when the cycle numberof irradiation with the laser beam 5 for the same zone of the nickellayer 2a is fixed at 10.

FIG. 12 shows that a larger amount of oxide film can be removed as theirradiation energy density of the laser beam 5 increases, and that theoxide film 4 can be similarly removed even when the initial thickness ofthe oxide film is changed.

FIG. 13 is a graph plotting, by way of example, the relation between thethickness of the oxide film 4 (nm) formed on the surface of the nickellayer 2a as ordinate and the laser beam irradiation cycle number for thesame zone or area of the nickel layer 2a as abscissa when theirradiation energy density of the laser beam 5 is fixed at 0.75 (J/cm²).

FIG. 13 shows that the thickness of the oxide film decreases as theirradiation cycle number increases.

EXAMPLE 3

FIG. 14 is a sectional view for illustrating a process for making anelectronic device such as semiconductor integrated circuit (LSI)according to Example 3 of this invention.

In Example 3, an oxide (or residues of organic matters, carbon, etc.) onthe surface of a nickel (Ni) layer (or nickel alloy layer) 2a formed onthe top of a ceramic substrate 1 is removed by the metal surfacetreatment procedure of the above Example 1 or 2, and then a platinglayer 10 is applied, as shown in FIG. 14.

The plating may be any of electroplating, electroless plating orreplacement plating, but the plating material herein is generally gold(Au) to prevent reoxidation.

The oxide film on the nickel (Ni) layer or nickel alloy layer 2a ofmetallized layer can thus be removed while preventing the layer fromreoxidation.

FIG. 15 shows a configurative sectional view of an electronic device towhich the anti-reoxidizing means of Example 3 has actually been applied.

In the process of Example 3, a nickel (Ni) layer (or nickel alloy layer)2a of metallized layer is formed on a ceramic substrate 1 and an organicinsulating layer 15 is applied thereon. This organic insulating layer 15is perforated to expose said nickel (Ni) layer 2a and an oxide film onthe surface of thus exposed nickel (Ni) layer 2a is removed by thesurface treatment procedure of the above Example 1 or 2, and thereafteran anti-reoxidizing plating layer 10 is applied. Then, an input/output(I/O) pin 12 is bonded by solder 11.

As a result of applying the anti-reoxiding plating layer 10 afterremoving the oxide on the surface of the nickel alloy layer 2a, a goodelectric connection can be achieved between the input/output (I/O) pin12 of a semiconductor integrated circuit (LSI) or the like and theceramic substrate 1.

The input/output (I/O) pin 12 and the nickel (Ni) layer 2a on theceramic substrate 1 can be electrically connected in good conditions bythe solder 11 without applying the anti-reoxidizing plating layer (Auplating) 10 within about one week after the oxide film 4 has beenremoved by laser beam 5.

FIG. 16 shows a case where an electronic device and a nickel (Ni) layer2a on a ceramic substrate 1 are directly electrically connected bysolder 11 without using an inlet/outlet (I/O) pin 12 shown in FIG. 15.

Conventionally, such a connection always required fluxes, but theprocess of Example 3 eliminates the necessity of fluxes.

EXAMPLE 4

FIGS. 17A and 17B illustrate a process for making an electronic devicesuch as semiconductor integrated circuit according to Example 4 of thisinvention, wherein FIG. 17A is a plan view and FIG. 17B is a sectionalview taken along the line XVIIB--XVIIB of FIG. 17A.

In the process of Example 4, a metal film 13 (for example, chromium(Cr), titanium (Ti)) with good adhesive properties to an organicinsulating or dielectric layer 15 is formed on the organic insulatinglayer 15 and a nickel (Ni) layer (or nickel alloy layer) 2a is appliedthereon, as shown in FIGS. 17A and 17B. Oxide (or residues of organicmatters, carbon, etc.) on the surface of this nickel (Ni) layer (ornickel alloy layer) 2a is removed by the surface treatment procedure ofthe above Example 1 or 2, and thereafter a gold (Au) ribbon or gold (Au)wire 14 is bonded by ultrasonic heat pressure technique.

Such bonding has been usually difficult on the nickel (Ni) layer ornickel alloy layer 2a due to the presence of oxide film on the surface,but good bonding can be achieved as a result of removing the oxide bythe procedure of the above Example 1 or 2.

Although the foregoing Examples include the metal surface treatment ofthe solder layer 3a or nickel (Ni) layer 2a, this invention is notlimited to these Examples but may be applied to various metals fromwhich oxide film or organic matters should be removed.

The laser beam energy may be suitably controlled depending on the natureor property of the metal material.

Although a pulsed laser beam was mentioned above as an example, similareffects can be achieved by continuous irradiation with a laser beamhaving a longer wavelength such as CO, lasers provided that appropriatecontrolling means is added to prevent the metal texture itself frommelting.

The laser irradiation may cause the metal texture on the surface tomelt, but it is permissible so far as it lasts briefly only.

EXAMPLE 5

FIG. 18 is a sectional view showing a configuration or structure of acircuit substrate on which has been pre-set an electronic device to besoldered by a manufacturing apparatus according to an embodiment of thisinvention, FIG. 19 is a perspective view showing a configuration orstructure of an apparatus for manufacturing electronic circuitsaccording to an embodiment of this invention, FIG. 20 is a sectionalview showing an inside or inner structure of a treating vessel, and FIG.21 lists examples of the liquid used for pre-setting an electronicdevice.

FIGS. 18 to 20 show an electronic device 21, a liquid 22 for pre-settingthe electronic device 21, lands 23 for electric connection 23, a circuitsubstrate 24, solder 25, a treating vessel 26, a pressure controllingsection 27, an oxygen content monitoring section 28, a temperaturecontrolling section 29, an electronic circuit substrate transferringsection 30, a controlling section 31, an electronic circuit substrate tobe treated 32, a carbon heater 33, a cooling plate 34, a gas introducingsystem 35, and a vacuum exhaust or evacuation system 36.

As shown in FIG. 18, the electronic circuit substrate structure to betreated 32 formed of an electronic device and a circuit substrate whichare to be soldered together according to the embodiment of thisinvention comprises the electronic device 21 such as an LSI providedwith solder bump terminals of solder 25 and pre-set on the circuitsubstrate 24 made from ceramic, glass or epoxy, etc. by the liquid 22for pre-setting the electronic device 21 thereon. The electronic device21 and the circuit substrate 24 are aligned or registered so that eachof the lands 23 on the circuit substrate 24 to be soldered and thecorresponding solder 25 on the electronic device 21 may be matched witheach other.

As shown in FIG. 19, an apparatus for manufacturing electronic circuitswherein the electronic circuit substrate to be treated 32 formed of theelectronic device 21 pre-set on the circuit substrate 24 is treated tobond solder 25 on the electronic device 21 with the lands 23 on thecircuit substrate 24 according to the embodiment of this invention asdescribed above comprises the treating vessel 26 for carrying outsoldering by heating, cooling or otherwise treating the electroniccircuit substrate to be treated 32 as shown in FIG. 18, the pressurecontrolling section 27 for controlling the evaporation rate of theliquid 22, the oxygen content monitoring section 28 for monitoring theoxygen content in a low-oxygen content or concentration atmosphereformed in the treating vessel 26, the temperature controlling section 29for the carbon heater 33 heating the electronic circuit substrate to betreated 32, the electronic circuit substrate transferring section 30 forautomating a series of transfer operations and the controlling section31 for automatically controlling the whole apparatus.

As shown in FIG. 20, the treating vessel 26 contains therein the carbonheater 33 for heating an electronic circuit substrate to be treated 32,and the water-cooled metallic cooling plate 34 for cooling the heatedcarbon heater 33 and the electronic circuit substrate to be treated 32.The electronic circuit substrate to be treated 32 is disposed andtreated on the carbon heater 33.

The gas introducing system 35 and the vacuum exhaust or evacuationsystem 36 are connected to the treatment vessel 26 to control thetreatment atmosphere inside the treating vessel 26.

The treating vessel 26, gas introducing system 35, vacuum exhaust system36 and heating means such as the carbon heater 33 constitute a reflowheating system.

Now, a soldering process using an apparatus for manufacturing electroniccircuits according to the embodiment of this invention is describedbelow.

At first, a precontrolled amount of the liquid 22 is fed onto thecircuit substrate 24 by a dispenser (not shown). This feed amount iscontrolled to be capable of covering the solders 25 and the lands 23 butnot to move up the electronic device 21 even by the surface tension ofthe liquid 22 or other action. Then, the electronic device 21 such as anLSI is mounted on the circuit substrate 24 so that the solder 25provided beforehand on the circuit substrate 24 may be aligned orregistered with the corresponding or respective lands 23 on theelectronic device 21 coated with the liquid 22.

The circuit substrate 24 carrying the electronic device 21 forms anelectronic circuit substrate to be treated 32 as shown in FIG. 18, whichis then placed in the electronic circuit substrate transferring section30 of the manufacturing apparatus of FIG. 19. The electronic circuitsubstrate to be treated 32 placed in the electronic circuit substratetransferring section 30 is transferred by a sort of robot, such as aprogram-controlled manipulator or transfer mechanism, onto the carbonheater 33 in the treating vessel 26, as shown in FIG. 20.

Then, the gas in the treating vessel 26 is exhausted or evacuated by avacuum exhaust system 36 formed of a rotary pump or the like and anon-oxidizing gas such as He, nitrogen (N) or a reducing gas such as amixture of hydrogen (H) and nitrogen (N) is introduced via a flow- andpressure-controllable gas introducing system 35 to once restore theinside of the treating vessel 26 to atmospheric pressure. If the oxygencontent (concentration) within the treating vessel 26 as measured by theoxygen content monitoring section 28 is not lowered to a predeterminedlevel (preferably 10 ppm or less), the above vacuum exhaustion and gasintroducing steps are repeated until the predetermined low-oxygencontent atmosphere is formed. The low-oxygen content atmosphere has aneffect of inhibiting the oxidation of the circuit substrate 24, lands ofthe electronic device 21 and solder 25 on the electronic circuitsubstrate to be treated 32 during heating.

After the low-oxygen content atmosphere has been formed in the treatingvessel 26, the electronic circuit substrate to be treated 32 is heatedby direct thermal conduction from the carbon heater 33 while theoccurrence of abnormalities is constantly monitored during heating. Thisheating is controlled at a temperature higher than the melting point ofthe solder 25 by the temperature controlling section 29. When themelting point of the solder 25 is 221° C., for example, the temperatureof the carbon heater 33 is set at 250° C.

When the heating starts, the liquid 22 used for pre-setting theelectronic device 21 on the circuit substrate 24 begins to evaporate. Ifit is desired to promote or suppress this evaporation, the pressure ofthe gas introduced to form a low-oxygen content atmosphere as describedabove may have been set lower or higher than atmospheric pressure. Afterthe solder 25 melts and soldering is completed, cooling water issupplied to the water-cooled metallic cooling plate 34 to cool theheated carbon heater 33 and electronic circuit substrate to be treated32, and then the electronic circuit substrate to be treated 32 is takenout by the substrate transferring section 30.

Now, a process for soldering the electronic device 21 to the circuitsubstrate 24 is specifically explained, which enables to omit thecleaning step after soldering by allowing the liquid 22 on theelectronic circuit substrate to be treated 32 to completely evaporate byusing the manufacturing apparatus according to an embodiment of thisinvention.

In one embodiment of this invention, a solder having a melting point of221° C. is used and the temperature of the carbon heater 33 is set at250° C.

As the liquid for pre-setting the electronic device 21 on the circuitsubstrate 24, a rosin-free alcoholic liquid as shown in FIG. 21 may beused, for example.

The liquid A shown in FIG. 21 is ethylene glycol having a boiling pointof 197° C. which is relatively lower than the melting point 221° C. ofthe solder 25 used. Such a liquid begins to evaporate as the temperatureof the electronic circuit substrate to be treated 32 rises after thecarbon heater 33 starts to heat it. In this embodiment which uses thesolder having a melting point of 221° C. and the carbon heater 33 risingup to 250° C., the liquid will have completely evaporated beforesoldering is completed. Once the liquid has completely evaporated andthe solder 25 has been completely molten to complete soldering, coolingwater is supplied to the cooling plate 34 to cool the heated carbonheater 33 and electronic circuit substrate to be treated 32 and then theelectronic circuit substrate to be treated 32 is removed or taken out bythe substrate transferring section 30.

If the electronic device 21 has been pre-set on the circuit substrate 24by the liquid having a boiling point relatively lower than the meltingpoint of the solder 25 used as described above, the soldering can becarried out without leaving any trace of the liquid used for pre-settingand therefore the cleaning step after soldering can be omitted accordingto this embodiment.

On the contrary, the liquid B shown in FIG. 21 is triethylene glycolhaving a boiling point of 287° C., which is relatively higher than themelting point 221° C. of the solder 25 used. Such liquid slowlyevaporates even if the temperature of the electronic circuit substrateto be treated 32 rises after the carbon heater 33 starts to heat it, andthe liquid will not completely evaporate and will be left aftersoldering is completed.

In order to solve this problem, the treating vessel 26 is evacuated tolower its inner pressure after soldering is completed. This allows theliquid to have completely evaporated before the electronic circuitsubstrate to be treated 32 is cooled by the cooling plate 34, andenables to omit the cleaning step after soldering similarly to the casewhich uses the liquid A.

The evaporation rate of the liquid A or B is regulated by controllingthe pressure in the treating vessel 26 so that the liquid which haspre-set the electronic device may be left until the solder melts duringsoldering but the liquid may have completely evaporated when solderingis completed by the molten solder. Since the liquid is left until thesolder melts, solder joints can be protected against oxidation and morereliable soldering can be carried out even in an atmosphere containingsome level of oxygen.

As apparent from the foregoing description, this invention allows oxidefilm on the surface of metal to be removed without using flux byirradiating the surface of the metal such as solder with a laser beamhaving a lower energy than the energy changing the texture of thesurface of the metal.

This invention also prevents the oxidation of circuit substrates, landsor contact pads of electronic devices and solder during heating andensures more reliable soldering by pre-setting an electronic device anda circuit substrate by a specific liquid and hot-melting solder in alow-oxygen content atmosphere.

Finally, this invention enables to omit the cleaning process using fluxsuch as chlorofluorocarbons, and thus enables to prevent harmfulinfluences on the global environment as well as to reduce productionequipment and production steps.

What is claimed is:
 1. A process for manufacturing an electronic circuitincluding disposing an electronic device on a circuit substrate, andhot-melting a solder formed on said electronic device or said circuitsubstrate to bond said electronic device and said circuit substrate,said process comprising the steps of:irradiating said solder with alaser beam, feeding a liquid onto said circuit substrate, aligning andmounting said electronic device at a desired position on said circuitsubstrate having been fed with said liquid, and hot-melting said solderin a low-oxygen content atmosphere to bond said electronic device andsaid circuit substrate and make said liquid evaporate.
 2. A processaccording to claim 1, wherein said liquid is a rosin-free alcoholicsolution.
 3. A process according to claim 1, wherein said step ofhot-melting said solder in a treating vessel further comprises the stepof:regulating an evaporation rate of said liquid by controlling an innerpressure of said treating vessel.
 4. A process according to claim 1,wherein said laser beam irradiation step further comprises the stepof:adjusting an energy density which allows said solder to melt.
 5. Aprocess according to claim 1, wherein said step of adjusting the energydensity of said laser beam further comprises the step of:adjusting theenergy density of said laser beam at a value between 0.5 J/cm² and 4.0J/cm².
 6. A process according to claim 1, wherein said laser beamirradiation step further comprises the step of:irradiating said solderwith said laser beam at 6 to 10 cycles per part of said solder.
 7. Aprocess according to claim 1, wherein said step of hot-melting saidsolder in a low-oxygen content atmosphere further comprises the stepsof:placing said circuit substrate in a treating vessel, forming alow-oxygen content atmosphere with a predetermined oxygen content insaid treating vessel, and hot-melting said solder in said treatingvessel.
 8. A process according to claim 7, wherein said step of forminga low-oxygen content atmosphere with the predetermined oxygen content insaid treating vessel further comprises the steps of:evacuating saidtreating vessel, and introducing a gas into said treating vessel.
 9. Aprocess according to claim 8, comprising the step of:introducing anon-oxidizing or reducing gas as said gas.
 10. A process according toclaim 1, further comprising the step of:cooling said circuit substrateafter said electronic device and said circuit substrate have beenbonded.
 11. A process for manufacturing electronic circuits includingdisposing an electronic component on a circuit substrate, andhot-melting solder bumps or a solder layer formed on said electroniccomponent or said circuit substrate in a treating vessel to bond saidelectronic component and said circuit substrate, said process comprisingthe steps of:irradiating said solder bumps or solder layer with a laserbeam, feeding a rosin-free alcoholic solution onto said circuitsubstrate, aligning and mounting said component at a desired position onsaid circuit substrate fed with said solution, placing said circuitsubstrate in said treating vessel, forming a low-oxygen contentatmosphere having a predetermined oxygen concentration within saidtreating vessel, and hot-melting said solder bumps or solder layer insaid treating vessel to bond said electronic component and said circuitsubstrate and make said liquid evaporate.
 12. A process according toclaim 11, wherein said laser beam irradiation step further comprises thestep of:adjusting an energy density of said laser beam at a level lowerthan an energy density which allows said solder bumps or solder layer tomelt.
 13. A process according to claim 12, wherein said step ofadjusting the energy density of said laser beam further comprises thestep of:adjusting the energy density of said laser beam at a valuebetween 0.5 J/cm² and 4.0 J/cm².
 14. A process according to claim 11,wherein said alcoholic solution is ethylene glycol or triethyleneglycol.
 15. A process according to claim 11, wherein said step offorming a low-oxygen content atmosphere with the predetermined oxygencontent in said treating vessel further comprises the stepsof:evacuating said treating vessel, and introducing a non-oxidizing orreducing gas into said treating vessel.
 16. A process according to claim11, wherein said step of hot-melting said solder bumps or solder layerin said treating vessel further comprises the step of:regulating anevaporation rate of said liquid by controlling an inner pressure of saidtreating vessel.
 17. A process according to claim 11, further comprisingthe step of:cooling said circuit substrate after said component and saidcircuit substrate have bonded.
 18. A process for manufacturing anelectronic circuit including disposing an LSI provided on a lower facethereof with a plurality of solder bumps on a circuit substrate, andhot-melting said solder bumps on the lower face of said LSI in atreating vessel to bond said LSI and said circuit substrate, saidprocess comprising the steps of:irradiating said solder bumps with alaser beam to clean surfaces of said solder bumps, feeding a rosin-feealcoholic solution onto said circuit substrate, aligning and mountingsaid LSI at a desired position on said circuit substrate, fed with saidalcoholic solution, to pre-set said LSI thereon, placing said circuitsubstrate in said treating vessel, forming a low-oxygen contentatmosphere having a predetermined oxygen concentration within saidtreating vessel, and hot-melting said solder bumps in said treatingvessel to bond said LSI and said circuit substrate and make saidalcoholic solution evaporate.
 19. A process according to claim 18,wherein the step of hot-melting said solder bumps in said treatingvessel further comprises the step of controlling a pressure in saidtreating vessel to regulate an evaporation rate so that said alcoholicsolution may complete evaporate off.
 20. A process for manufacturing anelectronic circuit including disposing an electronic component on acircuit substrate, and hot-melting a solder formed on said electroniccomponent or said circuit substrate to bond said electronic componentand said circuit substrate, said process comprising the stepsof:irradiating a connection of said circuit substrate or said solderwith a laser beam, feeding a liquid onto said circuit substrate,aligning and mounting said electronic component at a desired position onsaid circuit substrate having been fed with said liquid, and hot-meltingsaid solder in a low-oxygen content atmosphere to bond said electroniccomponent and said circuit substrate and make said liquid evaporateduring said hot-melting.
 21. A process according to claim 20, whereinsaid liquid is a rosin-free alcoholic solution.